Friction materials such as for disk braking pads, braking linings or the like are used for braking of vehicles. The friction material functions in braking by producing friction on a facing material such as disk rotor, braking drum or the like. Therefore, a superior friction coefficient, abrasion resistance (long service life of friction material), strength, sound and vibration properties (braking noises and abnormal noises are difficult to generate) and the like are required for the friction material. The friction coefficient is required to be reliable regardless of vehicle velocity, deceleration and braking temperature. In addition, there may be a case in which the friction material adheres to the facing material by rust generated at a friction interface, and problems such as abnormal noises at starting of driving, surface delamination of the friction material (rust delamination) and the like occur. In order to solve the problem of adhesion due to rust, a friction material composition is proposed in which zinc functioning as a sacrificial anode or an alkaline metal salt increasing pH is added (see Patent sacrificial anode or an alkaline metal salt increasing pH is added (see Patent Documents 1 and 2).
In the friction material, a friction material composition containing binder, fibrous substrate, inorganic filler, organic filler and the like is used, and in order to exhibit the abovementioned properties, a friction material composition containing one or more kinds selected from the above ingredients is generally used. In particular, copper is added to a friction material in the form of fiber or powder, and is an effective component for maintaining friction coefficient under braking conditions at high temperatures (anti-fade property), improving abrasion resistance at high temperatures, and improving strength of friction material. However, a friction material containing copper may generate abrasion powder containing copper during braking, and it may be a cause of contamination of rivers, lakes and oceans. Therefore, there is a tendency to limit use of copper.
Under recent circumstances limiting use of copper, the following Patent Document 3 discloses a technique of adding potassium titanate having multiple convex portions and biodegradable inorganic fiber as a method to improve strength and abrasion resistance in a composition not containing copper.
The Patent Documents are as follows:
Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-107026
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-107027
Patent Document 3: Japanese Unexamined Patent Application Publication No. 2013-076058.
The friction material not containing copper that is harmful to the environment has low material strength, and there is a problem of rust delamination. The rust-proofing effect proposed in the Patent Documents 1 and 2 and the strength improving technique of friction material in a composition not containing copper proposed in the Patent Document 3 are not sufficient to improve effects against rust delamination of the friction material not containing copper.